bonding plaster to alabaster

[Joseph Farina]

Hello Everyone,
Has anyone ever tried to make a plaster "addition" to a piece of alabaster? I have some experience with plaster, but very little with alabaster. I am assuming that the alabaster is mainly gypsum, the same material from which plaster is made. I know that permanent additions are possible when adding wet plaster to the surface of a dry plaster cast. The key point in that case is to make sure the dry plaster cast is fully saturated with water, and that it stays saturated for the full period of time during which the fresh plaster addition becomes solid. (If fresh plaster is added to a dry plaster cast, it will crumble off when dry, because the dry plaster will immediately rob the fresh plaster of its water, so that it can't go through the normal cycle of hardening.)

What I really had in mind was taking a piece of natural alabaster, cutting off part of the block, pulverizing it, and heating it to about 190C (374F) so that it hopefully becomes "plaster of paris." (That temperature was given for the making of plaster of paris from natural gypsum.) This "homemade" plaster would hopefully be similar in color and other properties, once re-hydrated with water, to the natural stone (I'm just guessing here). Then soak the natural alabaster, and keep it fully wet while the homemade plaster is applied to it and becomes hard. My goal is to make small, but permanent additions (for indoor use only) to alabaster.

Thanks

[C***@public.gmane.org]

use a cement bonding agent

[Don Dougan]

RE: ". . . make a plaster "addition" to a piece of alabaster?"

Hi Joseph,

I wouldn't rely solely on the strength of the bond between the plaster to
the alabaster by itself (but then I don't count on even epoxy bonds
without some kind of mechanical reinforcement) , but you can certainly do
what you are describing.
I cannot help but ask why?
I have done something similar to what you describe, but it was not for
making my own art — it was conservation/restoration work.

Though I have done repairs to many 19th century alabaster carvings that
were damaged through breakage, I usually employ tinted polyester resins
or epoxies as the most expedient method as per the client's budgetary
desires. However, I had been approached by a client who had several
alabaster and marble sculptures that had been damaged in a fire and then
by inexperienced help in 'rescuing' them. The house fire damage was
indirect — smoke and water damage — and then they were packed by the
insurance company in cardboard boxes with newspaper for packing . . .
with results you can imagine. Lots of little pieces in the bottom of the
cardboard box when I received them!

The client (whose circa 1865-80 period house was a museum showpiece)
wanted the works cleaned and repaired for display, and didn't want me to
use epoxy or polyester resins and fillers — she wanted the works repaired
the way they could have been a hundred years earlier. Given that I had
little chance of obtaining the same quality or pure colored alabaster as
the originals were made from, she agreed that plaster was the next best
option. After I had cleaned the pieces I used off-the-supplier's-shelf
plaster (#1 molding) for filling and then carving missing areas to
restore them. Luckily she had some photos of the pieces pre-damage as
reference for the missing areas.

I mixed the patches with a relatively high-proportion of plaster to water
to increase strength and minimize porosity, and I did use brass
escutcheon pins to reinforce the joints for the added patch portions,
drilling into the existing stone and applying the plaster patches over
the protruding half of the brass pin. Where I still had the broken
alabaster elements I used plaster as the cementing adhesive to reattach
them, a method that was employed in the 19th century by the carving
studios — though most often in adding portions or whole elements to make
"integral" bases for the carvings.

Though I did roughly shape the patches after the plaster was set, I
waited for the plaster to become bone-dry before attempting any
finely-detailed re-carving of the forms. This meant my repair work was
spread over the period of several weeks.

Of course the set and dried plaster is dead opaque and uniform in color
where the original alabaster was translucent and (though the originals
were all carved from high-grade white statuary quality alabaster) had
minor color variations, so a cosmetic application of color was requested
by the client. I used numerous very light coats of opaque paints and
then transparent washes to make the plaster areas blend, overlapping and
feathering slightly with the undamaged stone surrounding.
However, no matter how good the cosmetic blends look in full direct
sunlight or artificial spotlights, there are some backlit situations
where the repair simply looks dead compared to the natural undamaged
areas of stone. I suggested she consider directly illuminating the
pieces very carefully for display to minimize the backlighting problems.

If you are trying to conserve or repair something made in alabaster then
plaster will certainly work, but if you are making sculpture as a product
of your own studio I cannot help but wonder why not just carve additional
pieces of alabaster and fit or join them?

The conflicts arising from the concept of 'truth-to-material' and the
qualities of the joined alabaster/plaster cannot help but raise certain
questions in the viewer's mind. If raising questions is your goal then
why stop at mixing plaster and alabaster (i.e., why not try something
really unexpected like glass, ceramic or paper)?
On the other hand, if your goal is primarily one of process to produce
form that cannot be achieved simply through subtractive carving but
requires additive processes too, then perhaps you intend to make a mold
and cast the entire composite form into a cohesive whole material (i.e.,
such as bronze)?

I would be very interested in hearing your conceptual framework for this
idea of mixing plaster and alabaster. I am a firm adherent to the
proposition that everything is fair game in the field of making art — as
long as it works in terms of the dialog between artist and viewer (i.e.,
all's fair in love and war . . . and isn't art both?), so my curiosity is
boundless . . .

Good carving (and plastering) to you,
Don

www.dondougan.com

[Joseph Farina]

Hi Don,

Thanks a lot for your reply. Yesterday I did a quick test. I took a slice
of gray Italian alabaster (the kind available from Sculpture House) and put
it into a shallow tray with a little water. I did this in an effort to get
the alabaster as wet as possible before applying the plaster. Then I mixed
the plaster (Permastone) in the usual way, 3 parts to 1 part water, and
applied some to the alabaster.

I noticed, however, that the alabaster does not behave like a solid cast of
plaster in the sense that it doesn't "imbibe" the water like plaster does.
In other words, it doesn't seem nearly as porous as a hard plaster cast. So
instead of relying on the moisture in the alabaster to keep the plaster wet
during the hardening process, I had to increase the water level in the dish
so that there was a tiny "rivulet" of water going to the plaster to keep it
good and wet as the plaster hardened.

I removed it from the tray, let it dry overnight, and started to chisel it
off this morning. Sure enough, the areas that were not kept wet for the
hardening process came off without too much trouble, flaking off at the
plaster/alabaster interface. On the other hand, the areas which were kept
wet locked very tightly to the alabaster. In those areas, I couldn't remove
it at the interface (i.e., "flake" it off), rather, it behaved like a solid
material.

The next step is to see if it's feasible to make "homemade" plaster from
alabaster. The first obstacle is getting it into a powder form. My plan is
to use a large disk sander attached to a vacuum. Since alabaster is so
soft, I can grind it into a powder this way, and retrieve it from the vacuum
cleaner. Then heat the powder in an oven to somewhere between 212 and 374 F
(this temperature was given in Ralph Mayer's book). At this point, I don't
know how long this heating process should last, though. Then see if this
behaves like plaster when mixed with water.

I am planning to use this material in blocks of alabaster, in certain
central locations, below the front surface, to imbed another material into
the alabaster block. The additions will be rather small, less than one
square inch in cross-section at any given point, although the length of the
additions may become as large as 32 linear inches. Backlighting is not a
problem in this case, because the areas will be located centrally on the
face of the block, and be in recessed areas.

That's very, very interesting about 19th-century alabaster workers who used
plaster in combination with alabaster (I would like to know more about it).
Since both materials are gypsum, I would expect good results, and this was
confirmed in my test.

Joe

[d***@public.gmane.org]

Hi Joseph,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Your experiments are yielding some interesting results! It never occurred to me to try wetting the alabaster like you did, even though I am fully aware of and employ wetting plaster when joining to other plaster. Maybe my lack of preconception comes from having a weak background in chemistry while having a much stronger background in the physical mechanics of joinery. But live and learn.

RE: "That's very, very interesting about 19th-century alabaster workers who used plaster in combination with alabaster (I would like to know more about it)."

I have never done any specific research into the practice, just observations on many antique pieces that I have repaired over the years, and (more rarely) when I am using a salvaged piece of stone from a broken antique pedestal element for my own sculptural carving work.

I first noticed evidence of the practice upon statues that had an integral base appearing to be 'machined' — either a rectangular form with edges formed into roman-ogee/beaded-type moldings, or when the base form/ogee-molding was apparently turned on a lathe. Often these integral bases are enlarged by adding secondary or tertiary tiers that were affixed by cementing with plaster.

Of all the pieces I have worked-upon that had two or more elements still attached, perhaps 90% of those joined combined very-similar or alike pieces/colors of alabaster. Only a relatively few times have I observed contrasting colors/types of stone cemented/joined — though I have seen numerous contrasting stones employed as bases with gravity being the only method used to 'join' them. And of course I have seen a number of pieces that had evidence of once being attached to something else, but when I worked upon them they were simply solitary elements. For what it is worth, none I have ever examined showed evidence of any tinting used in the plaster, even when dark stones were being joined — but perhaps they just painted the finished joints afterward and I didn’t notice the traces.

In my observations where a secondary element was added from a separate piece of stone to increase the size of the base the original joint-cement was always plaster. Sometimes there would be a mechanical reinforcement as well, usually in the form of a reverse tapered hole (like a Lewis would go into) carved into corresponding locations in the two pieces being joined. This usually had a steel pin or bent wire, or — more rarely — a piece of brass wire cemented into the two reverse-tapered holes with plaster-filling. Often — but not always — the two adjoining surfaces were worked with a point; presumably to allow greater 'tooth' to the bonding plaster, but possibly simply because it was less work than smoothing the surfaces that were never meant to be visible.

Later I worked on couple of larger alabaster carvings of pairs of figures — scaled-down copies of "Eros and Psyche" by Canova. Cupid's wings were of course carved separately, and fitted with protruding brass keypieces that fit into slots carved in his back. The brass keys are cemented into the wings with plaster. As an interesting aside; over the years I think I have had to repair (or completely re-carve) either five or six different pairs of wings for various versions of the Eros theme — but that's a different story revolving upon the sagacity of the metaphor of wings made of stone! Anyway . . .

The Canova composition has Psyche lying on her back with Eros kneeling down as they embrace, so the base required for this sculpture needs to be relatively wide and deep in the horizontal plane. The carvings were about one-quarter life-size, so the dimensions (sans wings) were about twenty-four-inches high by thirty-six-inches wide by twenty-inches deep. That would require a large block of this relatively-pure white statuary-grade alabaster, and because of the nature of gypsum alabaster deposits, this size and quality of block is extremely rare — at least without hitting structural or visual flaws.

The carving studio(s) would naturally want to make the figures as large as possible, and the base only large enough to retain the structural integrity of the monolith (even if the base ought to be larger for the aesthetics of the composition). Since the aesthetic conceit for the base is a fairly regularly-structured pile of rocks (no molding forms here), the carving studio(s) originally making the copies used a second block of alabaster to make the bases sufficiently large enough for the 'aesthetically-correct' and ostensibly 'monolithic' composition. The somewhat irregular joints between the seemingly separate 'carved' rocks became the logical joint-line between the primary block of alabaster (containing the figures) and the secondary base block. The outer dimensions of the secondary base-block are both slightly wider and deeper than the primary block, and perhaps five or six-inches thick at their highest dimension.

In the first of the two very-similar "Eros & Psyche" copies upon which I worked, the joint between the primary and secondary blocks was relatively flat and level. In the other copy the primary block was slightly smaller and was obviously missing a chunk out of the bottom of the base. To accommodate this missing contour a third block was fitted between the two larger blocks — becoming a whole 'rock' in the final carving composition.

Both of the "Eros & Psyche" groupings used only plaster cementing to join the two or three blocks respectively, without obvious recourse to mechanical joints. It appeared that the final detail carving of the 'rockpile' was probably done post cementing. While I was cleaning the version with the third block (this was one of the carvings I mentioned earlier that had been damaged with smoke and water in a house fire), this third block came loose and required me to re-cement it with plaster (thus affording me some observational insights into the processes originally used). The large joints between the primary and secondary blocks remained sound throughout my work with the pieces, so it is purely reasoned supposition on my part that no further joining mechanism was employed other than the plaster cementing.

That is pretty much the sum total of what I can write about the practice of using plaster to join alabaster in 19th century stone carvings. Hope it helps inform your experiments. I do have some photos of some of the pieces I repaired (at least those shot after going digital a few years ago), though the detail shots are specifically of areas pre-and-post repair and unlikely to show very much of the plaster cement-joins.

If anyone else on the list has anything to add (or to help correct any erroneous interpretations) I would be very interested in reading them (or seeing photos off-list, of course) — including the results of your further experiments with the plaster and alabaster.
Good carving to you,
Don

Don Dougan
www.dondougan.com

[Marc Fields]

The other thing you can do is add an acryl to the plaster, increasing
its adhesive properties. Weldbond works really well for this
application. 1 part Weldbond to 3 parts water in your mix.

[Clive Murray-White]

Hi,

I may have come to this one a bit late but marble fireplaces used to be
stuck together with plaster.

I think the real point here is not whether you can stick plaster to stone
but what is trying to be achieved by doing so because its one of those
sculptural activities that demands real caution.

My guess is that it can only work, in sculptural terms, if both materials
are allowed to "be themselves" and do not pretend to be anything else, we
see plaster used as a patching material on some ancient sculptures,
replacing bits of broken noses etc, meaning that plaster has a kind of
context that travels with it wherever it goes. By my way of thinking as long
as you can find a kind of contextual precedent for the way you want to use
it, things will be fine. But if you cant the resulting object will scream
out there is something really wrong going on.

Good luck Clive
Sculptor Clive Murray-White

Web: http://www.cowwarr.com/CliveMurray-White/

[Joseph Farina]

Hi Don,

Thanks a lot for your reply concerning the use of plaster with alabaster.
Based on your experience as well as my test, I am pretty much convinced that
plaster has excellent adhesion to alabaster, if the plaster is kept
thoroughly wet for the full hardening cycle. This may take some ingenuity
in some cases, because I think the plaster should have some kind of
"connection" to a separate supply of water while it hardens. This is
especially important for small additions of plaster, when the alabaster
quickly takes moisture away from the setting plaster. For larger additions
such as the bases you mentioned, no such extra wetting may be needed, as the
large mass of plaster keeps its moisture for an adequate period of time. As
a possible method for keeping the plaster wet for small additions, a
container of water could be held in a high spot, with a piece of cloth
running down from the container to the plaster addition, to produce a
siphoning effect so that the plaster stays continuously wet. I think that a
proper bond made in this manner is probably the best kind of cement for
alabaster, since both materials are gypsum, and have the same kind of
thermal expansion. Also, I would presume that humidity cycling would have
similar effects to the plaster as well as the alabaster, but that's just a
guess. I do know from my test that the adhesion can be excellent.

Yesterday I tried to make some homemade plaster from alabaster. I took a
piece of the stone and rubbed it against some sandpaper to produce about a
tablespoon of powder. I heated it for 2 hours at 300F, then mixed it with
water in the usual way. It looked and behaved like commercial plaster,
except that this morning, it still hadn't dried out (it was still damp, and
this would not have happened with my other plaster). This may have been due
to inadequate heating of the powder. The castings were pretty soft, too,
but otherwise seemed the same as normal plaster. But the important thing
was that it retained NONE of the color characteristics of the dark gray
stone. It was completely white, just like ordinary plaster, even though the
alabaster was dark gray (almost black). Actually, looking back at it now,
I'm a bit embarrassed to think it would have been otherwise. Well, there it
is, it seems that no matter what color the alabaster, the plaster will turn
out white.

Joe